As is known, the hearth of a blast furnace is provided with an iron notch or tap hole through which molten iron is drawn off periodically--usually four or five times a day. The tap hole is plugged with clay or mud at all times except when the furnace is casting (i.e., except when the molten iron is withdrawn). When it is desired to tap the blast furnace, a pneumatic drill is used to drill into the clay to form an opening for the molten iron to be tapped.
The tap hole itself may be 5 to 9 feet long; and with presently-used drill bits, the time required to drill through the clay may be as high as 20 minutes, particularly in the case where the harder anhydrous clays are used. This is disadvantageous in a blast furnace operation where charging and tapping must be scheduled more or less precisely and where any delays are reflected as a loss in tonnage.